Integrated circuit packages (e.g., ball grid array packages, etc.) may include a substrate that can be similar to a small printed-circuit board. For example, the substrate may include several layers that are laminated together or otherwise attached to each other. The substrate can be used to route signals from a die or chip (or multiple chips) on the substrate to the balls on the bottom of the BGA package for connection to the outside world, for example via a larger printed-circuit board. The chips may include a plurality of bonding pads connected with bond wiring. The substrate may also include substrate traces in one or more routing layers of the substrate that provide electrical connections from the bond wires to the balls via one or more vias formed in the substrate.
Integrated circuits are decreasing in size. For example, semiconductor die sizes are becoming increasingly smaller, and in turn, die pad pitches are shrinking accordingly. Consequently, a relatively thin gold wire is needed for smaller die pad pitches. The cost of gold wire is a major contributor to the cost of producing a BGA. The gold content used in BGA production may be reduced by decreasing the gold wire length and/or the gold wire diameter. The wire length can be reduced by reducing the wire bonding finger pitch. However, this may result in wire bonding assembly yield loss and substrate cost increases due to the small finger pitches and traces pitch needed.
As the bond pad pitch decreases with the decreasing size of the die, the maximum wire diameter becomes limited. In addition, the maximum wire length is related to the maximum wire diameter. In general (e.g., for stability purposes), the thinner the bond wire, the shorter its maximum length becomes. Consequently, space for wire bonding on the substrate becomes an issue as the area surrounding the die becomes congested when shorter wires are used.
A cross-section of a conventional BGA package 100 is shown in FIG. 1A. A die 110 is on an uppermost layer 122 of the BGA substrate 120. Generally, the die 110 includes a plurality of bond pads (not shown) for connecting bond wires 150 to the substrate traces 121 on the upper surface of the uppermost substrate layer 122. The traces 121 connect the bond wires 150 to the ball bonds 140a-d, through one or more mechanical vias 130a-b, and one or more traces 127 on the lowermost surface of the lowermost substrate layer 126. However, it is difficult to mechanically drill vias relatively close to the die 110 because the diameter of the mechanically-drilled vias 130a-b is generally too large. This results in relatively high surface area of the substrate 120. Furthermore, as the distance between the mechanically-drilled vias 130a-b and the bond pads on the die 110 increases, the costs of packaging increase because more gold bonding wire is needed to make connections with bonding fingers (not shown) on the traces 121. Additional drawbacks of conventional BGA packaging approaches include the relative inflexibility of the placement of mechanically-drilled vias 130a-b in the substrate 120, discussed in further detail with regard to FIG. 3B.
FIG. 1B illustrates a top-down view of parts of the conventional BGA package 100 of FIG. 1A. Bond pads 115 may be positioned on the die 110 linearly, staggered, or in a concentric ring or other pattern. Bond wires 150 connect the bond pads 115 to the substrate traces 121 on the uppermost layer of the substrate 120.
FIG. 1C shows a cross-section of conventional BGA substrate 120. An insulative layer 128 may be formed over traces 121 and 160 on the uppermost layer 122 of substrate 120. Each bond wire (not shown) is attached or bonded to a bonding finger 161 exposed through an opening 111 in the insulative layer 128. The trace 160 connects one of the bond wires to a mechanical via 130 extending through the uppermost substrate layer 122, the middle substrate layer 124, and the lowermost substrate layer 126. One or more traces 127 electrically connect the mechanical via 130 to a ball bond (not shown) on a bump pad (not shown, but similar to bump pad 152 in the opening 151 in passivation layer 129) on the lowermost substrate layer 126. Traces 123 and 125 may be formed on substrate layer 124 on in substrate layers 122 and 126.
However, the conventional BGA substrate 120 of FIGS. 1A and 1C is relatively inflexible with regard to the placement of mechanically-drilled vias 130a-b. For example, referring to FIG. 1A, the number and the location of the mechanically-drilled vias 130a-b in the substrate 120 are restricted by the location of the ball bonds 140a-d under the lowermost substrate layer 126. Generally, the mechanically drilled vias 130a-b cannot be placed onto the ball area (e.g., opening 151 in the passivation layer 129 in FIG. 1C). As a result, options for routing the traces are also relatively inflexible.
Referring to FIG. 2, the diameter of the mechanical vias 230 (designated by the double-headed arrow labeled “A”) is relatively large, and therefore the mechanical vias 230 must be placed at least a minimum distance from the die 250. As a result, a relatively large surface area of the substrate is consumed, and longer bond wires for connection to the bond fingers 260 (e.g., through openings 211 in a passivation layer on the substrate) are needed. The length of the bond wires become an even more significant issue when one uses multiple rows of bond fingers (e.g., see distance “B” between die 250 and a second row of openings 212 for exposing bond fingers on the substrate). Furthermore, when a larger diameter wire is used, greater spacing between neighboring bond wires is also necessary.
FIG. 3 illustrates a cross-section of an alternative embodiment 300 of a conventional BGA package, in which laser vias 340a-b are formed in an uppermost layer 322 of a BGA substrate 320, and laser vias 345a-b are formed in a lowermost layer 326 of the BGA substrate 320. A die 310 is on the uppermost substrate layer 322. In general, bond wires 360 connect a plurality of bond pads (not shown) on the die 310 to substrate traces 321 on the upper surface of the uppermost substrate layer 322. The traces 321 connect the bond wires 360 to the balls 350a-b under the lowermost substrate layer 326 through one of the laser vias 340a-b in an uppermost substrate layer 322, one or more traces 323 on a middle layer 324 of the substrate 320, one of the mechanical vias 330a-b in the middle substrate layer 324, one or more traces 325 between substrate layers 324 and 326, one of the laser vias 345a-b in the lowermost substrate layer 326, and one or more traces 327 on the lowermost surface of the lowermost substrate layer 326. Locating the mechanical vias only in the middle layer 324, and the laser vias 340a-b and 345a-b in the uppermost and lowermost layers 322 and 326 of the substrate 320 may eliminate some of the drawbacks discussed in FIG. 1A. However, additional drawbacks result from the conventional BGA package 300 of FIG. 3, including an increased number of via formation processes and/or steps, possible limitations of current-carrying capacity and/or drive strength of a signal to or from an external package connection 350a-b to the die 310, etc., resulting in increased costs and decreased yields in the conventional BGA package 300.
This “Discussion of the Background” section is provided for background information only. The statements in this “Discussion of the Background” are not an admission that the subject matter disclosed in this “Discussion of the Background” section constitutes prior art to the present disclosure, and no part of this “Discussion of the Background” section may be used as an admission that any part of this application, including this “Discussion of the Background” section, constitutes prior art to the present disclosure.